For sure you can quickly hop back for lunch if your day is at the Magic Kingdom. All other parks I would plan for 45 minutes at least just to get back to your hotel. This includes walking from wherever you are in the park to the bus stop, waiting for the bus, riding the bus, then walking to your room. You're looking at 90 minutes just in transportation and you haven't even eaten anything yet. I would only recommend going back to the room if you are going to rest longer than just eating lunch. For example, eat, take a quick swim or nap, then go back out. If you are looking for a speedy lunch, heading back to the room is not a good idea.